1. Field of the Invention
The present invention relates to a liquid crystal display device (hereinafter, referred to an xe2x80x9cLCD devicexe2x80x9d) usable in, for example, computers and wordprocessors.
2. Description of the Related Art
Conventionally, active matrix LCD devices are known. An active matrix LCD device includes an active matrix substrate, a counter substrate, and a liquid crystal layer acting as a display medium held between the active matrix substrate and the counter substrate. The active matrix substrate includes switching devices such as thin film transistors (hereinafter, referred to as xe2x80x9cTFTsxe2x80x9d) and pixel electrodes which are both disposed in a matrix. The counter substrate includes a counter electrode and optionally a color filter. The active matrix substrate and a counter substrate are assembled together by a sealing member formed of a sealing material provided along perimeters thereof, with a prescribed gap retained therebetween.
Referring to FIG. 1, a plan view of an LCD device 100 is shown having an active matrix substrate 200, a counter substrate 300, and a sealing member 103. A xe2x80x9csealing portionxe2x80x9d is representatively depicted and is a three-dimensional region defined by the sealing member 103 (hatched area). Three-dimensionally, the sealing portion extends between a top and bottom of the LCD device 100 in a direction perpendicular to the sheet of FIG. 1. Two-dimensionally, the sealing portion is restricted by the sealing member 103. The term xe2x80x9ccell thicknessxe2x80x9d represents a distance between the active matrix substrate and the counter substrate after assembly.
In one known type of active matrix LCD device, the active matrix substrate includes an interlayer insulating layer for covering TFTs, gate signal lines for supplying a scanning signal for controlling the TFTs, and source signal lines for supplying a display signal. The pixel electrodes are disposed on the interlayer insulating layer. Thus, the pixel electrodes overlap the TFTs, the gate signal lines and the source signal lines. In such a structure, an area of the LCD device excluding an area having the TFTs, the gate signal lines and the source signal lines can entirely be used as an aperture for display. Thus, the numerical aperture is increased, thereby improving the brightness of the display.
The interlayer insulating layer is used for compensating for an uneven surface of a structure formed of the gate signal line generally having a thickness of about 0.3 xcexcm, the source signal lines generally having a thickness of about 0.3 xcexcm, and the TFTs generally having a thickness of about 1 xcexcm. The interlayer insulating layer is also used for reducing the parasitic capacitances between the pixel electrodes and the gate signal lines and the parasitic capacitances between the pixel electrodes and the source signal lines. The interlayer insulating layer is generally formed by coating a base plate, which has the above-described structure thereon, with an acrylic photosensitive resin film having a dielectric constant of about xcex5=4 by spin-coating. The interlayer insulating layer needs to have a thickness of about 3 xcexcm or more in order to achieve the above-described purposes.
When a film is formed by spin-coating, an error in film thickness of about xc2x15% is unavoidable. For example, when the average thickness of the film is set to be 3 xcexcm, the film thickness has a maximum deviation of 0.3 xcexcm.
The interlayer insulating layer is usually disposed in a sealing portion as well as the display area. The reason is that removal of the interlayer insulating layer from the sealing portion directly causes an error of 0.3 xcexcm in the cell thickness. In general, a tolerance regarding the cell thickness nonuniformity during the production process of LCD devices is 0.5 xcexcm. The error of 0.3 xcexcm, which corresponds to as much as 60% of the tolerance, makes the control of the cell thickness during the production process very difficult. Although the thickness of the interlayer insulating layer is nonuniform in a broad area, the cell thickness can be made uniform by providing spacers between the active matrix substrate and the counter substrate.
Elements such as, for example, the gate and source lines and the TFTs form an uneven structure having a varying height surface due to the height or thickness difference among these elements.
In LCD devices having no interlayer insulating layer, the cell thickness may be undesirably nonuniform because an active matrix substrate and a counter substrate have an uneven surface due to elements having different thicknesses, such as TFTs, electrodes and color filters. It is well known that such an undesirably nonuniform cell thickness can be avoided by optimizing the conditions of providing the above-described elements. In general, the cell thickness is not made undesirably nonuniform by a thickness difference of several micrometers among regularly arranged elements such as, for example, gate and source lines, pixel electrodes and TFTs. Rather, the cell thickness is made undesirably nonuniform by elements which are randomly arranged with a pitch of as great as several millimeters (e.g., patterns for connection of driver ICs and terminals for supplying signals to the counter electrode), since base plates of the active matrix and counter substrates, which are generally formed of glass, are distorted in correspondence with the varying heights of the elements.
Even in LCD devices including an interlayer insulating layer, the surface of the active matrix substrate is not completely flat. Since the height of the interlayer insulating layer from the glass base plate is substantially determined by the height of the elements underneath the interlayer insulating layer, the flatness of the top surface of the interlayer insulating layer is determined by whether the areas between adjacent elements are properly filled with the interlayer insulating layer. Especially in the sealing portion, the surface of the uneven structure becomes more uneven due to the patterns for connection of driver ICs and terminals for the counter electrode which are randomly arranged with a pitch of as great as several millimeters. Thus, the cell thickness is likely to be undesirably nonuniform.
In the sealing portion, the degree of influence on the cell thickness varies in accordance with the height difference between the elements. With respect to each of a plurality of sides of the active matrix substrate, a height difference of several micrometers between adjacent elements which occur perpendicularly to a given side of the active matrix substrate do not cause an undesirably nonuniform cell thickness as long as similar height differences are present substantially entirely in the sealing portion. In contrast, height differences of several micrometers between two adjacent elements which occur parallel to a given side of the active matrix substrate do cause an undesirably nonuniform cell thickness for the following reason. When the height differences occur parallel to a given side of the active matrix substrate, the substrates cannot deform. Accordingly, the cell thickness varies with the higher elements acting as a fulcrum of a lever. Especially when the elements closer to the display area are higher than the other elements, the nonuniformity of the cell thickness significantly exerts an adverse influence on the display quality.
According to one aspect of the invention, a liquid crystal display device includes a first substrate; a second substrate; a liquid crystal layer interposed between the first substrate and the second substrate; and a sealing member formed of a sealing material for confining the liquid crystal layer between the first substrate and the second substrate, the sealing member defining a sealing portion. The first substrate includes a plurality of gate signal lines, a plurality of source signal lines, a plurality of switching devices disposed in the vicinity of intersections of the plurality of gate signal lines and the plurality of source signal lines, a plurality of pixel electrodes respectively connected to the plurality of source signal lines through the plurality of switching devices, and an interlayer insulating layer provided below the plurality of pixel electrodes and covering the plurality of switching devices, the plurality of gate signal lines and the plurality of source signal lines. The second substrate includes a counter electrode opposing the plurality of pixel electrodes with the liquid crystal layer interposed therebetween. The first substrate includes a part of the interlayer insulating layer and a plurality of terminals covered by the interlayer insulating layer in the sealing portion. A maximum height of protruding portions of the interlayer insulating layer along a top surface thereof is about 0.5 xcexcm or less.
In one embodiment of the invention, the plurality of terminals are arranged so that a distance between two adjacent terminals is less than about 100 xcexcm in a direction parallel to a given side of the active matrix substrate.
In one embodiment of the invention, a maximum height difference among the plurality of terminals is about 0.5 xcexcm or less.
In one embodiment of the invention, the plurality of terminals include signal input terminals.
In one embodiment of the invention, the plurality of terminals further include dummy terminals.
In one embodiment of the invention, the interlayer insulating layer has a thickness of about 3 xcexcm or more.
According to another aspect of the invention, a liquid crystal display device includes a first substrate; a second substrate; a liquid crystal layer interposed between the first substrate and the second substrate; and a sealing member formed of a sealing material for confining the liquid crystal layer between the first substrate and the second substrate, the sealing member defining a sealing portion. The first substrate includes a plurality of gate signal lines, a plurality of source signal lines, a plurality of switching devices disposed in the vicinity of intersections of the plurality of gate signal lines and the plurality of source signal lines, a plurality of pixel electrodes respectively connected to the plurality of source signal lines through the plurality of switching devices, and an interlayer insulating layer provided below the plurality of pixel electrodes and covering the plurality of switching devices, the plurality of gate signal lines and the plurality of source signal lines. The second substrate includes a counter electrode opposing the plurality of pixel electrodes with the liquid crystal layer interposed therebetween. The first substrate includes a part of the interlayer insulating layer and a plurality of terminals covered by the interlayer insulating layer in the sealing portion. A maximum depth difference of the interlayer insulating layer along a bottom surface thereof is about 0.5 xcexcm or less.
In one embodiment of the invention, the maximum depth difference of the interlayer insulating layer along the bottom surface is less than about 100 xcexcm in a direction parallel to a given side of the active matrix substrate.
Thus, the invention described herein makes possible the advantage of providing an LCD device for preventing an undesirably nonuniform cell thickness.
This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.